1. Field of the Invention
The present invention is directed to a polarizing antenna for a magnetic resonance apparatus for at least irradiating into an imaging volume surrounded by the antenna, of the type having two longitudinal conductors or a multiple thereof that are arranged parallel to an axis with the same lateral spacing and are terminated with electrically coupled end rings, with the size or the length of the imaging volume being dependent on the spacing of two end rings terminating the longitudinal conductors.
2. Description of the Prior Art
In magnetic resonance tomography, the quality of the diagnostics is dependent on obtaining an artifact-free image of the pathology in the region of interest, referred to as the xe2x80x9cfield of viewxe2x80x9d. Dependent on the application, image acquisition areas of different sizes are needed. When only relatively small body regions are acquired such as, for example, parts of the head, the heart or a dislocated joint, then the region of interest is small, whereas a large imaging area is needed for acquiring the spinal column or in the framework of angiography exposures. Antennas of different sizes that enclose different imaging volumes within which an image can be acquired are employed for the implementation of these different examinations. This means that different magnetic resonance apparatus or accessory parts in the form of local antennas must be used for different examinations. Another problem is that, given employment of the antenna for reception of the radiofrequency signals as well, a large irradiation region has a negative influence on the signal-to-noise ratio. Volume coils that enclose the imaging volume or the field of view (for example, the brain cavity in fact image such a volume as well as possible. With a larger imaging volume, however, the volume coil receives a great deal of noise, which leads to a decrease of the signal-to-noise ratio and thus to a degradation of the image quality. Making the irradiation region, and thus the measurement volume, smaller, in contrast, leads to a clear enhancement of the signal-to-noise ratio, and thus to an improvement of the image quality, since the signals are influenced significantly less by basic noise. In comparison thereto, small antennas are optimized in view of the signal-to-noise ratio and a lower power consumption, but have only a small imaging volume. As a result of the antenna design, the examining physician is forced to employ a magnetic resonance apparatus with a large or small antennaxe2x80x94if availablexe2x80x94dependent on the type of examination. When, for example, only a magnetic resonance apparatus having a large antenna is available on site, then the disadvantages in view of the noise and, possible image artifacts deriving from an examination of a relatively small region of interest must be accepted. Another goal in the framework of such examinations is to minimize the radiofrequency power emitted into the patient given optimized system power, and to not exceed the limit values for the absorption of radiofrequency energy, referred to as the xe2x80x9cspecific absorption ratexe2x80x9d (SAR). Again limits are reached when a relatively small region of interest is to be examined but the existing antenna or volume coil covers a large imaging volume, since the patient is unnecessarily subjected to radiofrequency energy in regions outside the imaging volume that is actually required for the region of interest.
An object of the present invention is to provide a polarizing antenna of the type initially described that allows a flexible operation of a magnetic resonance apparatus to satisfy the different demands associated with different examinations.
This object is inventively achieved in a polarizing antenna of the type initially described wherein the longitudinal conductors have a number of end rings allocated to them with which the longitudinal conductors can be terminated in resonant fashion and that can be optionally connected in a resonant coupling connection with the longitudinal conductors for varying the imaging volume that can be irradiated.
The inventive antenna allows the size of the imaging volume to be varied by selectively connecting (cutting in) one or more end rings that terminate the longitudinal conductors. The attending physician thus can select the imaging volume in the way that is most expedient for the examination to be undertaken. There is thus the possibility of lengthening or shortening the antenna in steps dependent on the position of the end rings. The inventive antenna thus combines the advantages of a long antenna or volume coil with those of a short antenna or volume coil since the characteristic can be arbitrarily switched dependent on the application. The quantity of radiofrequency energy emitted into the patient can be minimized while simultaneously achieving a high image quality as a result of a suitable selection of the imaging volume required for the examination. An extremely flexible examination of the patient is thus available to the physician by a corresponding adaptation of the antenna design to the examination to be undertaken.
In an embodiment of the invention the end rings are arranged at a number of specific positions with reference to the longitudinal conductors. In this embodiment of the invention, the end rings are distributed along the length of the longitudinal conductors, i.e. the effective length of the longitudinal conductors can be varied by corresponding cut-in of one or two end rings between which the imaging volume is defined. Each connectable end ring can be composed of a number of transverse conductors that are arranged at the specific longitudinal positions between the longitudinal conductors and that can be coupled to the longitudinal conductors via switchable conductor connections. As needed, these transverse conductors of an end ring are coupled to the longitudinal conductors via the switchable connections and the termination of the longitudinal conductors is thus effected.
Inventively, the conductor connections be capacitor arrangements that form resonant locations and to which a switch means for opening and closing the connection is allocated. A resonant termination ensues in a simple way via the capacitor arrangements; the connection being opened or closed with the switch connecting or disconnecting for the end ring to the longitudinal conductors.
In one embodiment, the switch can be an impedance transformation circuit, with a separate switch allocated to each capacitor arrangement. This impedance transformation circuit, which is also called a xcex/4 circuit, makes it possible either to switch the capacitor arrangement into resonance in order to switch the end ring into a terminating connection, or to cancel the resonant condition and decouple the end ring.
Alternatively, the switch can be fashioned as an electromechanical switch, with a separate electromechanical switch allocated to each capacitor arrangement or a central switch for opening and closing the connections can be provided for each end ring. In this embodiment, the line connections thus are electromechanically switched, either centrally or decentrally.
In another embodiment, the switch can be a semiconductor switch, with a separate semiconductor switch allocated to each capacitor arrangement. Alternatively, a central semiconductor switch can be provided for each end ring, via which the line connections are opened or closed. Expediently, semiconductor switches in the form of PIN diode switches are employed.
Inventively, switchable end rings can be exclusively employed, i.e., each end ring allocated to the longitudinal conductors can be coupled to the longitudinal conductors as needed, so that this can be terminated by the respective end ring. In this embodiment, thus, two end rings must be cut in for the termination. In an alternative embodiment the longitudinal conductors are terminated with a non-switchable end ring in the region of one end, with the imaging volume being defined between this and one of the switchable end rings arranged distributed along the length. In this embodiment. This is especially advantageous, for example, where mainly head exposures ensue. The imaging volume can be defined between a non-switchable end ring arranged at the longitudinal conductor end, this representing a fixed resonant termination, and a switchable end ring arranged distributed over the length. In the case of a head exposure, for example when only the brain is to be examined, the end ring lying closest to the fixed end ring is cut in. A neighboring end ring is cut in instead of the first for an exposure of the entire head. A third end ring is cut in for the termination when, for example, the neck region also is to be acquired. In this case as well, the non-switchable end ring can be composed of a number of transverse conductors arranged between the longitudinal conductors at a common end region, these being connected to the longitudinal conductors via capacitor arrangements, which form resonant locations; in this case, however, the connections are not switchable, as stated. It should be noted that exactly two end rings are always needed to function for imaging, one or both thereof being switchable, as described.
The above-described embodiment is designed as a high-pass antenna. An alternative design of the polarizing antenna, which, like the first embodiment functions as a linearly polarizing antenna given employment of two longitudinal conductors, and as a circularly polarizing antenna given employment of at least four longitudinal conductors, is designed as a low-pass antenna. In this low pass embodiment at least one end of the longitudinal conductors is connected to a first end ring that is in turn followed by at least one further, neighboring end ring, and the longitudinal conductors can be terminated via each of the end rings. In this embodiment, each longitudinal conductor is xe2x80x9clengthenedxe2x80x9d with end rings. In order to enable this, it is inventively provided that the first end ring is composed of a number of first and second transverse conductors arranged following one another in the circumferential direction. Each first transverse conductors are connected to a longitudinal conductor via a capacitor arrangement and to the second transverse conductors via a switchable line connection. This end ring, or every end ring, is composed of further first and second transverse conductors connected via switchable line connections, the first transverse conductors thereof being connected via further capacitor arrangements to the first transverse conductors of the preceding end ring. The capacitor arrangements are designed such that, dependent on the end ring that is cut in, the capacitor arrangements that are activated form the resonant capacitance. The longitudinal conductors, thus, are lengthened by the first transverse conductors of the respective end rings, with these first transverse conductors being connected to the longitudinal conductors or to one another via capacitor arrangements. These series-connected capacitor arrangements are designed such that the required resonant capacitance always occurs regardless of which end ring is cut in. The cut-in of the individual end rings ensues with the switchable connections between the first and second transverse conductors of each and every end ring. When, for example, the second end ring is to be cut in, then the connections of the first end ring are opened; only the first transverse conductors are coupled to the longitudinal conductors via the capacitor arrangements. At the second end ring, in contrast, the connections are closed, so that this end ring has a terminating connection to the longitudinal conductors. Of course, more than two end rings can be provided in the described way.
A separate switch can be provided in each switchable connection, or a central switch can be provided for the switchable connections of each and every switchable end ring. The switches can be mechanical switches or semiconductor switches, preferably in the form of PIN diode switches. As in the first embodiment, all switch types that enable a dependable and defined cut-in of an end ring can be employed.
Here as well, switchable end rings can be exclusively provided, i.e. both ends of the longitudinal conductors have a number of end rings in the above-described form. Alternatively the longitudinal conductors can be terminated with a non-switchable end ring in the region of one end, with the imaging volume being defined between this end ring and a switchable end ring provided at the other end.
Inventively, the longitudinal conductors and/or the transverse conductors can be fashioned as ribbon conductors or rod conductors. The conductors are arranged on a cylindrical surface, the cylinder of this surface coincides with the axis of a cylindrical imaging volume surrounded by the antenna.
Even though single-piece longitudinal conductors can be employed, it has proven expedient, for suppression any eddy currents that may be induced in the longitudinal conductors, for the longitudinal conductors to be composed of a number of conductor parts that are arranged parallel side-by-side and proceed in longitudinal direction, and that are connected via capacitor arrangements enabling a flow of high-frequency currents. The capacitor arrangements typically have a capacitance between 1 and 20 nF.